As for fuel cell generation systems of small sizes, or of the class of several kW or less for the household use or the like, most systems are of a normal pressure working type in which gas is supplied at normal to low pressures from the viewpoint of safety or with the intention of improving efficiency by reducing power for auxiliary components. Many of such systems use fuel cells of the solid polymer type in view of a possibility of cost reduction on one hand and the fact that the working range is low in temperature (60 to 80 degrees C.) on the other.
In the solid polymer type fuel cell generation system, direct current power is generated as a fuel gas such as hydrogen and an oxidizer gas such as air are supplied to a fuel electrode and an oxidizer electrode of a fuel cell, respectively, and both gasses react electrochemically between both electrodes. The ratio of the amount of gas actually consumed for the reaction to the amount of gas supplied is called the utilization ratio. A low utilization ratio means that a large amount of gas is wasted and that the generation efficiency of the system is low. Too high a utilization ratio poses the risk of causing deterioration of cell constituting materials. Therefore, the utilization ratios are controlled to appropriate values for both fuel and oxidizer.
In the conventional fuel cell generation system, the utilization ratio is controlled to be constant as shown in FIG. 9. In other words, the amounts of fuel and oxidizer supplied are controlled to be in the following relation:Supplied fuel gas flow rate=Consumed fuel gas amount÷Preset fuel utilization ratio (constant value)Supplied oxidizer gas flow rate=Consumed oxidizer gas amount÷Preset oxidizer gas utilization ratio (constant value)Here, the supplied fuel gas flow rate or the supplied oxidizer gas flow rate is related to the DC load current as shown in FIG. 10.
With the conventional fuel cell generation system described above, however, decrease in performance or local deterioration of materials occasionally occurs, in particular when gases are supplied at low pressures or the fuel cell is operated at an output that is lower than the designed rated operating point.
Another problem is that component devices, for operation at low outputs, are required of a wide range of turn-down ratio, which inevitably invites increase in cost. Because of the above, with the solid polymer type fuel cell generation system for the low pressure operation, the turn-down ratio at a low load has been restricted and the operating range has been small.
Therefore, the object of this invention is to provide a generation method using a fuel cell generation system that is less likely to invite decrease in performance or local deterioration of materials and that does not require a wide turn-down ratio for component devices, and the fuel cell generation system itself.